Since about 1990 all commercially available diode lasers (edge-emitting semiconductor lasers) have been epitaxially grown as separate confinement (SC) heterostructures. A SC heterostructure, in a simplest form, includes an active layer often referred to as a quantum-well (QW) layer bounded on each side by a waveguide layer (optical confinement layer). Bounding each waveguide layer is a doped cladding layer (electrical confinement layer). The cladding layers are doped to increase electrical conductivity of the layers. One of the layers is p-doped and the other n-doped. This heterostructure is, electrically, a p-i-n diode. The heterostructure is typically grown on an n-type semiconductor wafer beginning, after any intermediate layers, with the n-doped cladding layer. The completed structure is usually mounted “p-side down” on a heat-sink.
SC heterostructures are typically symmetrical structures. That is, the waveguide-layers have the same thickness and are of the same semiconductor material composition, and the cladding layers have the same thickness, and with an exception that dopants are different, also have the same semiconductor material composition (different from that of the waveguide layers).
The p-doped and n-doped cladding layers are connected, sometimes via one or more other conductive layers, to respectively positive and negative terminals of a source of electrical current. Forward electrical bias applied from the source causes injection of holes from the p-side of the junction and electrons from the n-side of the junction into the depletion region in which lie the (un-doped) waveguide and quantum well layers (the active region of the heterostructure). The injected holes and electrons re-combine in the QW-layer, generating radiation having a wavelength characteristic of the material of the QW-layer. The waveguide layers have a higher band-gap (lower refractive index) than that of the QW-layer and the cladding layers have a higher band-gap (lower refractive index) than that of the cladding layers.
The heterostructure is terminated at one end thereof by a multilayer reflector that is highly reflective at the wavelength generated by the QW-layer. At the other end of the heterostructure is a multilayer having a relatively low reflectivity, or no multilayer at all, which also provides a relatively low reflectivity. This causes the QW-layer and the bounding waveguide-layers to function as a relatively low Q, waveguide laser-resonator. Output radiation is delivered from the low-reflectivity-end of the resonator.
SC heterostructure lasers are characterized in particular by a threshold-current density, which is the minimum areal current density which must be passed through the heterostructure to cause onset of lasing action. The threshold-current density increases with operating temperature of the laser. As threshold-current increases, overall efficiency decreases. Maintaining a low temperature increases the cost and complexity of cooling arrangements. Further changing the temperature of a heterostructure can be useful for changing (tuning) the output radiation wavelength of the heterostructure. For these reasons alone there is a need to reduce the temperature dependence of the threshold current.